My observation of 30% better mileage is ONLY based on comparison with the V10 Class C I just traded up from (which had the light truck version of the Triton). It got 7.5-8 on the highway (I checked mileage on every tank) unless I was towing the boat (4,200 lb.), which cost 1 mpg. It also didn't like to go much over 55-60; not the engine but the general feel of stability and noise level.
My FRED averaged 9.8 mpg on my trip from NC to ID, driving 65-75 into a daunting headwind for all but the last 300 miles. That comes out to exactly 30% better.
So, I know for a fact that *I* have seen a 30% improvement over my last rig but I have no idea how it would stack up against the same coach and chassis with the RV version of the Triton.
At the end of the day, for me, the mileage is really irrelevant and certainly no justification for choosing the diesel because, driving it about 5K mi/yr like we do, it will only save about $300/yr in fuel, so it would take 32 years to recover the cost difference between the Cummins and the V10. I'm just pleased that I have a larger motorhome that gets better mileage than what I was getting before with a smaller one.